LOAD-INTERACTIVE STEERED-INDUCTOR DC-DC
CONVERTER WITH MINIMIZED OUTPUT FILTER
CAPACITANCE
S. M. Ahsanuzzaman1, Amir Parayandeh1, Aleksandar Prodić1, Dragan Maksimović2
1
. ECE Department, University of Toronto, Toronto, CANADA
2
. ECEE Department, University of Colorado at Boulder, USA
Abstract— The paper introduces a concept of loadinteractive digitally controlled point-of-load (POL) dc-dc
converters where the size of the output filter capacitor can
be minimized based on the enhanced load-related
communication between the digital load and the converter
controller. Based on the information about upcoming load
changes, the inductor current of the dc-dc converter is
adjusted such that the converter output current matches
the new load, thus minimizing the energy storage
requirements for the output filter capacitor. The
effectiveness of this approach is verified on a digitally
controlled 3.3 V/ 30 W experimental prototype, where the
dc-dc converter is a steered-inductor non-inverting buckboost. Transient response results show that, compared to
standard converters with near-time optimal control, the
new system has about three times smaller voltage
deviation, allowing for a proportionally significant
reduction in the output capacitor size.
I.
INTRODUCTION
In recent years, advanced “power aware” digital chipsets
have been introduced in devices such as cell phones,
computers, and other electronic equipment having a large
portion of digital hardware. These digital loads communicate
with power supplies and, to a certain extent, regulate their
operation to reduce the overall power consumption [1-4].
Examples include dynamic/adaptive voltage scaling
(DVS/AVS) and adaptive body biasing (ABB) techniques,
where the supply voltage of the digital signal processors is
dynamically changed, depending on the processing load, to
minimize dynamic and static power consumption [1-6]. Also,
the latest Intel chipsets, provide the switched-mode power
supply (SMPS) with an indication of the current demand it
expects over a period of time [7], so that this information can
be used for phase shading that improves efficiency of multiphase SMPS, as shown in [8]. Methods that dynamically
perform efficiency optimization of a segmented power stage
based on the prediction of the load current from a digital
signal stream processed by the electronic load have also been
shown [9]. So far, communication between the digital load
and the SMPS, has mostly been motivated by power savings.
The objective of this paper is to show how this interaction
can further be utilized to minimize the overall size of the
SMPS, by dramatically reducing bulky and costly energy
storage output filter capacitors. The novel system of Fig.1
consists of a modified non-inverting buck-boost converter, i.e.
steered-inductor buck-boost, a digital controller, and a
communication block providing load current information.
The modified buck-boost converter has an additional
switch SW5, for current steering [12], which, as it will be
shown here, provides dramatic improvements of transient
response in both buck and boost modes. It should be noted that
the non-inverting buck boost is a common stage in numerous
battery powered applications (e.g. [15]), so compared to those
systems, no additional switches or conduction losses are
introduced.
To provide the load current information, i.e. pre-load
command, a block similar to those implemented in the latest
generation of Intel processors [7] is used. Based on the preload command, during heavy-to-light load transients, the
power stage is reconfigured, to steer the inductor current away
from the output capacitor into the source. Similarly, the load
information is used during the light-to-heavy transient to ramp
up the inductor current to the upcoming load value before the
transient occurs.
Fig.1: Digital Controller with modified buck-boost converter
This work has been supported by the Natural Science and Research
Council of Canada (NSERC)
978-1-4244-4783-1/10/$25.00 ©2010 IEEE
980
II.
PRINCIPLE OF OPERATION
In a point-of-load (POL) dc-dc converter, the bulky output
filter capacitor is sized based on the energy storage required to
supply current during large step-load transients. This capacitor
size is usually significantly larger compared to the size
required to meet the output voltage ripple specification. The
latency in the inductor current change, i.e. its limited slew
rate, compared to that of the load is the major factor in
determining the value of the output filter capacitance.
The converter of Fig. 1 is a modification of the steeredinductor topology proposed in [12]. This topology also
resembles the conventional non-inverting buck boost
converter [13], except for an addition of SW5. This switch
allows flexible and bidirectional control over the slope of the
inductor current, which, in conjunction with the load
information communicated by the load, as described in the
following subsections, is a key element in providing dramatic
improvements in the dynamic response. In the buck mode,
SW4 is on, SW3 is off at all times while SW1 and SW2 change
their states at the switching rate. When operating as a boost,
SW1 is on, SW2 is off at all times and switches SW3 and SW4 are
active.
It should be noted that constant operation in buck-boost
mode (instantaneous conduction of SW1 and SW3 followed by
the conduction of SW2 and SW4), is also possible. In this case,
it is avoided in steady state operation, to minimize the number
of active switching components operating at switching
frequency and, consequently, to reduce the switching losses.
Furthermore, in the buck mode, the rms current and
consequently, the conduction losses, are also reduced.
A. Light-to-Heavy Load Transient Improvement
When operating as a conventional buck, the rate of change
of the inductor current during a light-to-heavy load transient is
limited by:
diL Vg − Vout
=
dt
L
,
(1)
where Vg is the input voltage, Vout is the output voltage and L
is the converter inductance. As a result, during a sudden lightto-heavy load change, the output capacitor initially discharges
providing the difference between the load and output currents,
until the inductor current reaches the load value.
In the boost mode, when the input voltage is relatively low
(compared to that of the output), the slope of the inductor
current during a light-to-heavy load transient is limited by
diL Vg
=
dt
L ,
(2)
which causes a latency in the following of the load current.
Moreover, to respond to a load transient, a fast acting
controller initially disconnects the inductor from the load,
leaving the output capacitor to provide all the current. In a
properly designed SMPS, this capacitor is usually selected to
Fig.2 Light-to-Heavy Steer Inductor Operation
be large enough, so that the charge lost during transients does
not significantly affect the output voltage.
In the system introduced here, the converter switches in
the steering/boost mode of operation, based on a pre-load
command. In this mode SW1 and SW3 are conducting to
increase the inductor current. In some cases, this action is
periodically replaced with the conduction of switches SW1 and
SW4 to maintain the output voltage regulation, as described in
the following section. During the steering/boost mode, the
controller increases the inductor current to the load level prior
to the load change, such that, ideally, at the transient instant,
the two currents are matched. This results in a minimum
amount of charge supplied by the capacitor. As shown in
Fig.2, during the pre-charge phase, the inductor current is
charged at the maximum instantaneous rate equal to Vg/L.
B. Heavy-to-Light Load Transient Improvement
During a heavy-to-light load transient the inductor current
slew rate of the buck converter is limited by:
diL − Vout
=
L ,
dt
(3)
In modern low power devices, where the output voltage of
the converter is often very low (0.8-1.2V) [14], this limit
severely slows down recovery from heavy-to-light load
transients causing the output voltage overshoots. To minimize
the overshoots, in a conventional converter, the output filter
capacitor must be oversized.
In a traditional boost the situation is similar; the slope of
the inductor current, limited by
di L V g − Vout
=
dt
L
,
(4)
causes the capacitor overcharge. This is a significant problem
when the difference between the input and output voltage of
the converter is small, which is often the case in numerous
portable applications, where a non-inverting buck-boost is
used to slightly boost the battery voltage [16].
Unlike the light-to-heavy load transient operation that
requires charging up the inductor current prior to the load
transient, the heavy-to-light load operation is performed at the
point when the actual load transition occurs. At that instant,
the controller closes SW2 and SW5, as shown in Fig. 3, to
discharge the inductor current to the new steady state value
while driving the current back to the source. It can be seen that
this action not only disconnects the inductor from the
capacitor and prevents overcharging but, in the buck mode,
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Fig.3 Heavy-to-Light Steer Inductor Operation
also changes the current slew rate to a significantly higher
value:
di L − V g
=
L
dt
(5)
In this case, the preload command supplies the controller
with the information about the new steady state current value,
which is used to determine the point when the regular buck or
boost operation could resume.
III.
PRACTICAL IMPLEMENTATION
The controller architecture, shown in Fig.1, incorporates PID
compensator that is used for steady-state voltage regulation
with a block named mode control logic. The mode control
logic is a finite state machine (FSM), which chooses different
operating modes for the converter, depending on the pre-load
information and the conditions in the circuit.
The state transitions of the FSM are shown in Fig. 4.
During the steady state, a modified PID controller operates the
converter as a buck or a boost, depending on the input and
output voltages (as in [17]). Once the ‘Pre-Load’ command is
received, the mode control logic determines the required steerinductor current action. In the case of a light-to-heavy load
transient, steering/boost is activated prior to the occurrence of
the load step, to achieve the required inductor current. Once
the desired current is reached, the FSM generates the ‘Done
Signal’ to inform the load that the power consumption can be
increased. In the case of the heavy-to-light load transient, SW2
and SW5 are turned on to steer the inductor current back to the
source Vg.
Fig.4. State flow diagram of the mode control logic
A. Preventing Significant Capacitor Discharge During
Inductor Current Steering
As shown in Figs. 2 and 3, during the inductor current
steering, the output capacitor is not connected to the inductor.
As a result, it discharges through the load that causes a voltage
drop. When the load change is significant, this voltage drop
cannot be neglected. In order to minimize this variation, a
threshold for the maximum allowable voltage dip is set, the
output is monitored, and a periodic switching between
steering and boost operation is activated accordingly.
The operation in this mode is illustrated in Fig.6. In the case
of light-to-heavy load transient, if the output voltage drops
below the threshold (Vout < VThreshold), the mode control logic
momentarily stops the steer operation to charge up the output
capacitor back to the desired level by turning on SW1 and SW4.
After the voltage is recovered, the logic reestablishes the
steering through SW1 and SW3. This allows increasing the
inductor current while maintaining the output voltage.
Similarly, during a heavy-to-light load transient in order to
prevent capacitor discharging beyond the threshold the steer
operation is periodically paused and the output capacitor is
charged by the inductor by turning off SW5 and turning on SW4
(Fig. 4).
Fig.5 demonstrates the operation of the mode control logic.
The logic governs the inductor current steering after receiving
the ‘Pre-Load’ command during steady state. Once the
operation is completed the “Done Signal” is created to inform
the load that the inductor current has reached the new
requested value. It can also be seen that for the changes of
load from a very light to a heavy value (as shown in Fig. 5) the
steer operation reduces to a single switching action, where
only SW1 and SW3 are turned on until the desired current is
reached. Such simplified operation is possible as long as the
inductor current can be charged to the new steady state value
before a significant change of the capacitor voltage due to the
load current occurs. Otherwise, a more advanced control
action is required, as described in the following subsection.
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Fig.5. Gating signals for different modes of operation.
Fig.8. The light-to-heavy load transient of the proposed load-interactive
controller– Ch1: output converter voltage (100mV/div); Ch2: inductor current
(3.5A/div).
To verify the operation of the system shown in Fig. 1 an
experimental system was built. The 30-W, 3.3-V, 500 kHz
power stage is designed for the input voltage ranging between
1.5V and 12V. To compare the performance, a time optimal
controller capable of recovering output voltage through a
single on-off action, similar to [10], was developed as well.
The experimental results presented in Figs. 7 to 10 show
comparisons of the transient responses of both converters and
verify superior performance of the load-interactive steeredinductor topology. The waveforms show the comparison for
the case when the converter is operating as a buck with a 10V
input and 3.3 V output. Fig. 11 and 12 show the converter
switching in steer and boost operation mode.
Fig. 7 shows the transient response of the time-optimal
controller to a 6.5A light-to-heavy load transient. The gating
signals of SW1 and SW2 depict the action of the time-optimal
control during the transient. Fig. 8 shows the response of the
load-interactive system. It can be seen that the new system has
less than one half of the voltage undershoot for the same load
step. Moreover, unlike the time optimal controller, the loadinteractive system exposes the inductor to a much smaller
peak current preventing possible saturation. The pre-load
command that informs the converter prior to the occurrence of
the load step is also shown. Based on the pre-load command
SW1 and SW3 are turned on, to charge up the inductor to the
desired level. Once the desired current is achieved the regular
Fig.7. The light-to-heavy load transient of the time-optimal controller–
Ch1: output converter voltage (100mV/div); Ch2: inductor current
(3.5A/div).
Fig.9. The heavy-to-light load transient of the time-optimal controller– Ch1:
output converter voltage (100mV/div); Ch2: inductor current (3.5A/div).
Fig.6. The converter operation as a boost converter during light-toheavy load transient.
IV.
EXPERIMENTAL SYSTEM AND RESULTS
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Fig.10. The heavy-to-light load transient of the proposed load-interactive
controller– Ch1: output converter voltage (100mV/div); Ch2: inductor
current (3.5A/div).
PID operation is resumed and it continues until the next preload command is received.
Figs. 9 and 10 show the transient responses of the timeoptimal controller and that of the proposed load-interactive
converter to a 6.5A heavy-to-light load step, respectively. A
similar improvement to that for the light-to heavy transient
case can be observed. The waveforms of Fig.10 also
experimentally verify the operation described in Section II. It
can be seen that the steer inductor operation during the heavyto-light load transient is performed at the point of the actual
load step. The digital gating signals of SW2 and SW5 indicate
that the inductor current is driven back to the source until it
reaches the desired value.
Figs. 11 and 12 show the converter operation in steer and
boost mode, due to the output voltage drop below threshold
voltage. In Fig. 11 it is shown that the inductor current level
Fig.12. Boost converter like switching during steer and boost operation– Ch1:
output converter voltage (100mV/div); Ch2: inductor current (5A/div).
achieved is below the desired load current level due this
operation. Nevertheless, this operation brings the inductor
current close to the desired level, while maintaining the output
voltage in the acceptable range. Fig. 12 zooms into this
operation to show the boost converter like switching. As this
operation is taking place during buck mode, when SW1 and
SW4 conduct to restore the output voltage, the inductor current
increases with a slope (Vg-Vout/L) which is lower than the slope
during the steer operation (Vg/L).
CONCLUSIONS
A load interactive flexible digitally controlled dc-dc
converter with minimized output filter capacitor is introduced.
It is shown that by using a modified buck-boost converter,
named steered inductor buck-boost, and by improving
communication between the converter and its digital load the
size of the output capacitor can be drastically reduced. Based
on the information about the load current, the inductor current
is steered into or out of the capacitor, at a much higher slew
rate compared to the conventional designs, minimizing the
current provided by the capacitor during load transients.
Theoretically, the newly proposed system opens a possibility
of designing dc-dc converters such that the output filter
capacitor is selected only based on the maximum allowable
switching ripple, thus drastically reducing the overall size of
the dc-dc converters.
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